Traditionally, fluids such as medications are injected into patients, either subdermally or intradermally, using hypodermic syringe needles. The body of the syringe is filled with the injectable fluid and, once the needle has pierced the patient's skin, the syringe plunger is depressed so as to expel the injectable fluid out of an opening in the needle. The person performing the injection is usually a trained medical services provider, who manually inserts the hypodermic needle between the layers of a patient's skin for an intradermal injection, or beneath the skin layers for a subcutaneous injection.
Intradermal or subdermal delivery of a medication through the use of a hypodermic needle requires some skill and training for proper and safe administration. In addition, the traditional method of intradermal injections requires actual physical contact and penetration of a needle through the skin surface of the patient, which can be painful for the patient. Traditional needle injectors, such as hypodermic syringes, are also expensive to produce and difficult to use with prepackaged medication doses. Needle injectors also suffer from increased danger of contamination exposure to health care workers administering the injections, and to the general public when such injectors are not properly disposed of.
Needle-less injectors are designed to obviate some or all of the aforementioned limitations associated with hypodermic needle syringes; although in overcoming such limitations, needle-less injectors present difficulties of their own. One such difficulty is the removal of air or gas from that portion of a needle-less injector that contains the injectable product (e.g., a pharmaceutical solution), especially when the amount of air or gas contained in that portion is or becomes substantial. At least three mechanisms may give rise to air or gas in that portion of a needle-less injector that contains the injectable product: (1) the physiochemical separation of dissolved gas from the injectable product (e.g., during storage of a needle-less injector pre-filled with an injectable product), (2) the introduction of air into the needle-less injector by turbulent filling conditions (e.g., during introduction of an injectable product into the needle-less injector), and (3) loading a needle-less injector with an insufficient quantity of the injectable product to adequately fill the injector.
Regardless of the mechanism responsible for the presence of air or gas in a needle-less injector, subdermal hematomas, tissue damage, and scarring from mechanical force injury may result when pockets of such air or gas are present in an injector prior to dispensing the injectable product contained therein. Within the optimal range for acceleration of liquid medication through the skin via a needle-less injector, liquid readily penetrates the skin while air or gas does not. Thus, air or gas pockets accelerated against the skin lead to the formation of a bruise and can be quite painful for the recipient, whereas liquid medication passes into and/or through the skin without discomfort.
Furthermore, when a cap is removed from the end of a pre-filled needle-less injector, exposing the dispensing area for application to the skin surface, any air or gas pocket not already situated at the dispensing end may tend to migrate toward that end, due to the pressure change caused by cap removal. This motion of the air or gas pocket may force some of the injectable product from the needle-less injector, thereby diminishing the volume of the product that will be injected into the recipient. This may render the dosage level inaccurate, if a nontrivial volume of the product is lost from the injector prior to use.
In the context of injection by more traditional means such as with a preloaded syringe, it is well established that any significant amount of air or gas in such a device will cause pain for the recipient and potentially far more dire consequences if the amount of air is substantial. Air or gas pockets may develop in these syringes much in the way described above with regard to needle-less injectors, as these devices are frequently subject to similar filling and storage conditions. However, those administering traditional injections can readily obviate these limitations by evacuating air from the liquid-containing chamber of a syringe by partially depressing the plunger while the syringe is inverted immediately prior to administration of an injection. This is generally not possible with conventional needle-less injectors, as the entire volume of a needle-less injector ampoule is generally evacuated in one step during normal operation.
Notwithstanding the possible development of an air or gas bubble therein, conventional, pre-filled needle-less injectors generally contain a fixed amount of an injectable product, and the entire fixed amount of this injectable product is injected upon actuation of the needle-less injector. For instance, if a conventional needle-less injector contains a 0.5 ml dosage of a pharmaceutical product, then the entire 0.5 ml volume is generally administered to a recipient of the injection upon a one-time actuation of the needle-less injector. This fixed attribute of conventional needle-less injectors presents difficulty to those for whom the volume of a pre-filled needle-less injector does not correlate with their physiological or therapeutic needs.
This problem generally manifests in one of two types of injection recipient, although the need to adjust the volume of a needle-less injection is by no means limited to these scenarios: (1) injection recipients for whom the pre-filled volume of a needle-less injection is too large, and (2) injection recipients for whom the volume of required injections varies over a period of time. For example, a 0.5 ml volume of a pharmaceutical product may be suitable for the treatment of a disease condition in an average adult, yet a child may require a significantly smaller dosage. Alternatively, particular disease conditions may be treated with periodic injections of a pharmaceutical product, yet the required bolus size of these injections may change depending on a variety of factors. The treatment of diabetes with regular insulin injections is an example of such a disease condition and corresponding therapeutic scheme; different volumes of insulin injections are required depending upon, among other factors, blood glucose level.